Reduction of radiated spurious signals is a criterion of modern communication receivers and transmitters, particularly in low and zero intermediate frequency architectures that facilitate decreased equipment costs. In such radios, a local oscillator leakage cannot be removed by radio frequency filters. A local oscillator to radio frequency isolation of a mixer used to shift the radio frequency to or from an intermediate frequency becomes important.
Regulatory bodies specify tight criteria for spurious radiation. A general specification for receivers by the European Committee for Standardization and Communications (ETSI) limits the leakage of local oscillator radiation to −55 decibel-milliwatts (dBm) to avoid interference to nearby receivers. The performance criterion of low and zero intermediate frequency receivers dictates even lower levels of spurious radiation. Radiated spectral mask specifications also impose a high local oscillator to radio frequency isolation in low and zero intermediate frequency transmitters.
Conventional mixers use a fundamental local oscillator signal to heterodyne the radio frequency to or from the intermediate frequency. The specified isolation of the high-level local oscillator is difficult to achieve due to a dependency on a balance of transformers. An alternative architecture is a sub-harmonic mixer that uses antiparallel diodes to implement a mixer where the high level local oscillator signal is an even sub-harmonic of the radio frequency.
It would be desirable to implement an alternating anti-parallel diode mixer structure.